There are numerous standard computer peripherals available that allow "computer graphics" to be generated electronically. Typically these take the form of vector or raster displays with the input means provided by some form of touch tablet on which the user can draw and see the results of such work in real time on an electronic display.
The use of high resolution optical scanners to generate electronic, continuous tone raster image signals to depict continuous tone originals such as graphic art work and photographs is also well known. Usually the raster image signals are used in electronic plate making equipment to modulate a scanning laser beam, an electronic beam in a cathode ray tube, or the like, to fashion a facsimile of the continuous tone original on an appropriate printing master. The content of the original must be in a final form before the scanner generates the raster signals because limitations in the means to for making changes to the electrical image like touch up work, corrections, or modifications of artwork by an artist.
Normally, an artist changes shading, changes tone levels, corrects or modifies color hue, and erases or smears small defects. An artist also varies the shape of the tip of the implement used to render various artist effects, as well as filling in details, erases regions, and otherwise changes and modifies the artistic work. Such image retouching, when done electronically, is provided by devices which can be manipulated as if it were a brush or other implement by an operator viewing a cathode ray tube or other type display, to make corrections, modifications, or erasures to an electrical raster image during real time interaction between the user and the displayed image.
Such memory devices store the electrical signals representative of an image sought to be corrected or modified. The electrical image is made visible to an observer by providing the electrical signals to a display device. A brush tip image, for example, is created on the display over the visual image. The electrical brush usually includes a pointer device that is manipulated by the user to alter or touch up the continuous tone image. The pointer modifies the tone of the displayed image when it is moved in a brush like fashion and a button the pointer is depressed. Such movement of the pointer causes a like movement of the brush tip image. The motion and button depression allow the user to obtain image changes under the brush tip image with a stroking action which, with significant limitations, approaches that of an artist working on a painting.
In one well known embodiment, the pointer is activated by what is conventionally known as a mouse. The apparatus includes a ball bearing which is rolled over a surface to generate positive signals that move the image of the brush over the visual image on the display. The pointer may also include a plurality of buttons which are used to touch up images by simultaneously rolling the pointer over the surface and depressing one on the buttons to increase tone level of the signals in the memory corresponding to the signals in the region of the brush; to decrease the tone level image signals in the region of the brush; or to vary the size of the image of the brush tip when it depressed along with one of the other buttons. Numerous variations are available, such as hand held pointers, and varying the size or type of implement. Cursor and pointer devices are also well known in the prior art as well as systems and methods to modify rather than replace existing image data.
A fairly common application of digital techniques comprises an electronic graphics system with a touch tablet having a stylus, a computer, and a framestore with associated color generation capacity for the display. The user draws with the stylus of the touch tablet and the computer registers the coordinates (x,y) of the stylus while recalling the selected color with which the user has chosen to draw. The computer then provides the appropriate addresses to the framestore where the pixel at that address is modified to hold a pre-selected code corresponding the chosen color which it receives as incoming data. When the framestore is read at normal broadcast video rates then the lines, or illustrations drawn by the user are visible on the display.
It is also well known to generate color for display from RAM stores providing the blue, green or red components respectively to generate a desired color combination, where equal amounts of the red, green or blue (RGB) components will produce a monochrome image of a particular density. If data from the frame store is 8 bits wide, this allows for 256 different color combinations. The capacity for the RAMs is chosen accordingly. Various color parameters are directed into the RAMs from the computer and can be updated and varied as desired. Usually the RAMs are selected to operate as ROMs in dependence on the framestore generating output.
This type of system where the path from the touch tablet to the framestore and the display via the computer is all unidirectional, since the computer only writes from the framestore and does not read from it, and consequently makes no use of the information contained in the framestore. The quality of the product produced by such a system is significantly limited and is partially a result of the only 256 possible combinations in each pixel and therefore only 256 saturations, hues or luminance representations are possible on the screen for any given picture. To achieve a level of representation approaching "fine art" far more color combinations would be necessary.
Numerous prior art attempts to simulate drafting tools and to improve on the quality of the graphic product exist. Representative of this genre is U.S. Pat. No. 4,633,416 issued to Walker where color is replaced as the system draws rather than overlaying dyes using interpolation techniques. To achieve smearing effects a recursive low-pass filter is utilized. Significant limitations of such system include the inability of such system to render each bristle of a brush stroke independently; no are means provided to properly space dabs of paint to create an effective brush stroke; real time parameters such as velocity and stroke direction as controllers for the brush stroke are not provided, nor are provisions made varying the width or the shape of the brush stroke.
Various other patents describe attempts to simulate the rendering of printed articulation. Representative of this genre include U.S. Pat. No. 4,524,421 issued to Searby et al., an U.S. Pat. No. 4,514,818 issued to Walker, U.S. Pat. No. 4,751,503 issued to Kermisch, U.S. Pat. Nos. 4,775,858 and 4,931,956 issued to Stapleton, U.S. Pat. No. 4,318,096 issued to Thornburg et al., and U.S. Pat. No. 3,486,826 issued to Mueller. None of the prior art, however, provides a method or system for accurately rendering an accurate electronic analogy of artists tools such as paint brushes, charcoal, chalk, pencils, or other implement. Nor are any systems or methods described in which critical factors in producing a very high quality product are disclosed such as dye-concentration color mixing; effective variable grain penetration; provisions allowing for receptor grain penetration to affect the strokes in real time during the stroke; means for varying the shape of the tip of the brush across the stroke in order to accurately simulate a brush stroke; or means for rendering a brush stroke in multiple strokes for the purpose of simulating brush bristles.